1. Field of the Invention
The present invention relates to electrodeionization and more particularly, the invention relates to an electrodeionization substrate and a device for treating fluids via electrodeionization.
2. Description of the Related Art
Prior to their ultimate use, feed streams must often be pretreated to remove unwanted ionic contaminants. Typical clean-up processes include the use of ion-exchange resins and electrodialysis. In ion-exchange, after the targeted ions are removed from a feed solution, the ion-exchange resins are exhausted and have to be regenerated by acids, bases, or salts. Thus the process produces an equivalent or higher amount of waste salt stream from the regeneration process.
Electrodialysis is an electrically driven ion-exchange membrane based process where by using a stack of alternating cation, anion, or bipolar membranes, ions are re-moved from a feed solution and purified and concentrated in a product or concentrate solution. Since the transport of the ions are done by electric power, electrodialysis processes do not consume equivalent quantities of acids, bases, or salts and do not produce a salt waste stream. When the ion concentration in the feed stream is low, i.e. below 0.5 to 1%, electrodialysis processes become unattractive because the low ionic conductivity in the dilute feed stream leads to very low flux and high energy consumption.
Electrodeionization (EDI), also known as xe2x80x9celectrochemical ion-exchangexe2x80x9d, at is an advanced ion-exchange technology that combines the advantages of ion-exchange and electrodialysis. In electrodeionization processes, ion-exchange resins are sequestered in dilute feed compartments to increase the ionic conductivity, so that even with an ionically dilute feed, a stable operation with higher flux and lower energy consumption than electrodialysis, becomes possible. The electric power also splits the water (H2O) to H+ and OHxe2x88x92 ions and the resins are thus regenerated while the ions are removed.
EDI technology is increasingly being used to make deionized water for boiler feed and high purity industrial water applications. There are also many other potential uses of such technology for deionization of organic process streams in the food processing and chemical industries. These uses will not need the removal of ions to low parts per million or parts per billion levels as is required for high purity water production. However, these process streams will have multiple types of ions, other contaminants and potential foulants, and high concentrations of organics, which cannot be lost from the feedstream. Thus, the EDI devices for these applications must be easily dissembled for frequent cleaning. Also, process economics require that there is virtually no leakage between the product feed and the salt concentrate that is removed.
Many configurations and devices have been patented for electrodeionization. Almost all of them have cation and anion exchange membranes flanking loose ion-exchange resins or beads. In order to prevent the escape of these beads, a wide variety of and confining/sealing methods are employed. For example, in U.S. Pat. No. 4,804,451 a very complex configuration of a spacer element is described wherein the cation and anion membranes are bonded by special adhesives to the spacer element to form a pocket. The anion and cation exchange resin beads are confined within these complex pockets. A very complex assembly of the spacers and membrane resin pockets are then put together to reduce leakage between the dilute and the concentrate compartments.
In U.S. Pat. No. 4,956,071 the membranes are compartmentalized via evenly spaced ribs to which the membranes are bonded with loose ion exchange resins filling these pockets. A complex assembly of these compartments are put together to prevent leakage between the dilute and the concentrate compartments.
Patents also exist (U.S. Pat. Nos. 4,747,929 and 5,681,438) describing complex spacer construction configurations incorporating attached membranes and loose ion exchange resins interspersed between.
U.S. Pat. No. 5,346,924 describes a method for making a non-porous ion exchange membrane using an ion exchange resin, and binders such as polyethylene of linear low density or high density are described. These non-porous membranes are then described in an electrodeionization assembly with loose ion exchange resins in compartmentalized pockets that are similar to the previously described patents.
Another U.S. Pat. No. 5,308,467 describes an apparatus that creates an ion exchange material from mono-filaments of cation and anion exchange material by radiation grafting and this ion exchange material can then be assembled between ion exchange membranes to make an electrodeionization apparatus.
EDI devices typically are utilized as a final polishing step for already ultra-pure water. As such, fouling of the rather complex compartmentalization and flow channels of EDI systems is relatively rare. Indeed, such systems are usually sealed upon manufacture inasmuch as the need for disassembly to facilitate cleaning is nil. Most of the current configurations develop small leaks from the dilute/feed compartments to the concentrate compartments. Whereas this is not a significant economic penalty for the production of ultra-pure water, such leaks cannot be tolerated for use with organic feedstreams where such losses from the feed would be uneconomical.
In light of the foregoing, none of the current ED devices provide for having simple assembly and disassembly to facilitate cleaning and reuse. Also, none of the current EDI devices provide for virtually leak-free conditions between the feed compartment and the concentrate compartment. Such optimal characteristics are required for EDI devices used to treat process streams high in organic material content, such as corn syrups, glycerol and others.
Ion exchange beads that are commonly used for EDI applications consist of strongly acidic containing sulfonic acid groups, or strongly basic containing quatemary ammonium groups. Other resins such as those with weakly acidic (carboxylic acid) or weakly basic (amines) groups are also used when required. These beads are cross-linked polymers usually styrenexe2x80x94divinyl benzene or acrylates. The resins can be gel type or macro-reticular type. Usually equivalent mixtures of cationic and anionic resins are used in the EDI compartments. For specialized applications one type of resin or adsorbent beads mixed with ion-exchange resins may be used.
The complexity of the current EDI devices primarily come from the need to confine the loose ion exchange or other adsorbent beads between the membranes while keeping very close contact amongst these beads and between the beads and the membranes. In one case that gets away from the beads (previously cited U.S. Pat. No. 5,308,467) a complex radiation grafting process with mono-filament of ion-exchange material is disclosed.
A need exists in the art for a porous immobilized ion-exchange material. The material should be readily adaptable to current EDI stack configurations. Also, a need exists for an EDI device incorporating porous immobilized ion-exchange material whereby the device minimizes leakage of feed and also minimizes contamination of feed with any concentrate waste stream formed. The material and device should not have ion-exchange particle leakage even at high flow rates, and the material should be regenerable in situ. The material also should be produced with common substrates.
It is the object of the present invention to provide a porous ion-exchange material and a device incorporating the material which overcomes many of the disadvantages of the prior art.
Another object of the present invention is to provide a porous but immobilized ion-exchange material. A feature of the invention is that standard ion-exchange particles are combined with a binder material to immobilize them while also maintaining the molecular characteristics (such as porosity and internal surface area) of the particles. An advantage of the invention is conferring a high degree of ionic conductivity between individual particles of the standard material while also allowing high throughput of the treated liquid stream.
Yet another object of the invention is to provide an economical method for subjecting feed streams to electrodeionization. A feature of the invention is the utilization of a porous, immobilized ion-exchange material which provides ion-conductivities higher than the feed stream. An advantage of the invention is the ability for the material to regenerate in situ.
Briefly, the invention provides a porous immobilized ion-exchange material comprising ion-exchange resins having cation-exchange moieties and anion-exchange moieties; and a means for immobilizing the moieties relative to each other while conferring ion-conductivity and liquid permeability to the material.
Also provided is an electrodeionization device comprising a cation-exchange membrane; an anion-exchange membrane juxtaposed co-planarly to said cation exchange membrane; porous ion-exchange material positioned intermediate said cation-exchange membrane and said anion exchange membrane to form a compartment, whereby the material comprises anion-exchange entities and cation exchange entities immobilized relative to each other via a binder which comprises at least 20 weight percent of said material; and a means for applying an electrical potential to said compartment, wherein the entities are embedded in thermoplastic selected from the group consisting of low linear density polyethylene, high density polyethylene, and combinations thereof. In one example, binder is present in a weight ratio to the entities of 1:3.
The invention also provides for a method for subjecting a fluid to electrodeionization, the method comprising supplying a porous, ion-exchange material wherein the material comprises anion exchange entities and cation exchange entities immobilized relative to each other; applying an electrical potential across the material; contacting the fluid to the substrate so as to facilitate removal of ionic contaminants from the fluid; and simultaneous with the step containing the fluid to the substrate regenerating the resin, in situ.